In addition to the antigens present in the ABO System, red blood cells sometimes have another antigen, a protein called the Rh factor.
- If the Rh factor is present, the PERSON's blood group is RhD positive;
- If the Rh factor is absent, the PERSON is RhD negative.
This means that a PERSON can be one of eight blood groups:
- A RhD positive (A+)
- A RhD negative (A-)
- B RhD positive (B+)
- B RhD negative (B-)
- O RhD positive (O+)
- O RhD negative (O-)
- AB RhD positive (AB+)
- AB RhD negative (AB-).